Method and device for the heat treatment of workpieces

ABSTRACT

The present invention involves a method and a device for the heat treatment of workpieces, which are heated with radiant heat generated through the combustion of gaseous fuel. Some of the workpieces are subjected to a carburisation atmosphere formed by a heated mixture of hydrocarbon containing gas and carbon dioxide, where the carbon dioxide has been separated from the exhaust gas of the gaseous fuel combustion. Other workpieces may be subjected to a nitrocarburisation atmosphere formed by a mixture of ammonia and the exhaust gas remaining after the carbon dioxide has been separated. By recycling some of the products of the combustion process, the present invention improves the efficiency of the carburisation process while reducing pollutant emissions.

FIELD OF THE INVENTION

The present invention relates to a method and a device for the heattreatment of workpieces, wherein the workpieces are heated with radiantheat which is generated through the combustion of gaseous fuel, inparticular natural gas, and wherein at least some of the workpieces aresubjected to a carburisation atmosphere.

BACKGROUND OF THE INVENTION

Such carburisation methods and devices known from practice have arelatively high energy requirement. Thus, those skilled in the art arecontinually striving to improve the cost-effectiveness of carburisationprocesses. At the same time, the environmental protection requirementshave also to be considered. Great importance is attached to thereduction of pollutant emissions. However, such emission reductionmeasures are cost-intensive and thus run counter to the efforts toimprove cost-effectiveness.

SUMMARY OF THE INVENTION

The object of the present invention was to optimise the carburisationprocess under these aspects, i.e. to reduce the pollutant emissions andat the same time to improve process control at least to such an extentthat there is no appreciable negative effect on cost-effectiveness.

This object is achieved by the method according to the present inventioncharacterised in that carbon dioxide is separated out of the exhaust gaswhich forms during the combustion of the gaseous fuel, that the carbondioxide is mixed with hydrocarbon-containing gas, in particular withnatural gas, and that the gas mixture is heated to produce thecarburisation atmosphere.

The impact on the environment is quite considerably reduced through theremoval of the carbon dioxide from the exhaust gases of the radiantheating system. The share of carbon dioxide in the exhaust gas isapprox. 11%. Most of said carbon dioxide can be removed. With theappropriate process control the remaining exhaust gas only contains lessthan 1% carbon dioxide after the treatment.

The carbon dioxide is fed to the carburisation process as a supplier ofoxygen and carbon with the effect that the carburisation time isconsiderably reduced, namely by 20 to 40%. The carburisation timedepends on the temperature, the diffusion coefficient and the masstransfer coefficient. At a given temperature the two latter coefficientsgovern the speed of the carburisation process, in the case of small ormedium-sized carburisation depths (0.2 to approx. 0.8 or 1.0 mm) saidcoefficients having equal ranking. The present invention has aparticularly favourable effect in this range. It leads to an increase inthe mass transfer coefficient by a factor of roughly 2.5.

A particular advantage of the present invention consists in the factthat the entire process can be operated continuously, it being possible,if necessary, to place the carbon dioxide removed in an intermediatestore.

The carburisation atmosphere is generated by an endothermic reaction.The heat necessary herefor is preferably removed from the radiant heatwhich is used for heating the workpieces. This can, for example beachieved by introducing the mixture of carbon dioxide andhydrocarbon-containing gas directly into the furnace chamber. However,with this procedure there is the danger of inadmissible soot formationoccurring. Therefore, it is more advantageous to pass the gas mixtureover a catalyst which ensures that the endothermic reaction can takeplace without the formation of soot. The catalyst also ensures optimalintermixing of the components. The endogas generator can be arrangedoutside the furnace chamber. However, then a separate heating system isgenerally necessary. Therefore it can be more advantageous to arrangethe generator in the furnace chamber, preferably in the furnace roofarea, i.e. where a high temperature prevails and also where the fans arelocated.

A further embodiment of the present invention proposes that the carbondioxide is removed from the exhaust gas formed during combustion of thegaseous fuel by changes in pressure. This method makes use of thepressure-dependent attachment properties of carbon dioxide, for example,to molecular sieves. It can be easily integrated into the continuousprocess and is low in cost.

Carbon dioxide and water form during the carburisation reaction. As thisreaction proceeds particularly rapidly with the method according to thepresent invention, a local excess of reaction products may occur leadingto undesired surface zone oxidisation of the workpieces. In order tocounteract this effect the present invention proposes that thecarburisation atmosphere be injected with heavy hydrocarbon. Theslow-reacting methane, which is preferably used in the form of naturalgas, is particularly capable of blanketing the products of thecarburisation reaction and preventing oxidisation of the material. Atthe same time it is ensured that the carbon level in the carburisationatmosphere is maintained. To prevent surface zone oxidisation it must beensured that the heavy hydrocarbon can make its way to the workpieces soas to shield the endangered surfaces. Here a local hydrocarbonconcentration of 4 to 6% should be established.

The exhaust gas from the radiant heating system remaining after theremoval of the carbon dioxide can be used as purging gas, for examplefor inertisation of locks. A major embodiment of the present inventionproposes that said remaining exhaust gas be used for nitrocarburisingpart of the workpieces, with the addition of ammonia. Normally, it iscustomary to use not only ammonia but also bought-in nitrogen andbought-in carbon dioxide for nitrocarburisation. With the presentinvention the two latter constituents are provided by the remainingexhaust gas in the process. This further improves cost-effectiveness andthus leads to a considerable increase in the desired optimisationeffect. The nitrocarburisation process can be readily integrated intothe continuous process as a whole. The removal of carbon dioxide fromthe exhaust gas of the radiant heating system is set so that both therequirements of the carburisation process and those of thenitrocarburisation process are taken into account. This gives a processwhich is both extremely cost-effective and extremely environmentallyfriendly.

Furthermore, the present invention provides for a device for the heattreatment of workpieces with at least one furnace chamber, which isprovided with gas-operated radiant heating tubes, and with a generatorfor producing carburisation gas for the furnace chamber, wherein saiddevice is characterised in that the radiant heating tubes are connectedby their exhaust gas pipes to a pressure-change device and that thepressure-change device is connected by its carbon dioxide outlet pipe tothe generator. The pressure-change device removes carbon dioxide fromthe exhaust gas of the radiant heating tubes, whereupon the carbondioxide enters the generator as a supplier of oxygen and carbon to reactthere endothermically with a carbon-containing gas, preferably naturalgas. For the purposes of heating the generator, it is preferable forsaid generator to be arranged in the furnace chamber, namely in the roofarea thereof.

A major embodiment of the present invention proposes that thepressure-change device be connected by its remaining exhaust gas outletpipe to a second furnace chamber which has an ammonia inlet pipe andserves to nitrocarburise part of the workpieces which are not to becarburised. The joint control system ensures that the individualprocesses are synchronised with each other and run continuously.

Gas-operated radiant heating tubes are also generally used for heatingthe second furnace chamber. It is particularly advantageous to alsoconnect their exhaust gas pipes to the pressure-change device so thatthe exhaust gas is subjected to the same treatment as the exhaust gasfrom the radiant heating tubes operating in the carburisation chamber.

Combinations of the features according to the present invention whichdeviate from the combinations discussed hereinbefore shall be deemed tohave been disclosed as essential to the present invention.

BRIEF DESCRIPTION OF THE DRAWING

The present invention will now be described in greater detail with theaid of a preferred embodiment of a device according to the presentinvention and the attached drawing. The drawing shows a schematic blockdiagram.

DETAILED DESCRIPTION OF THE INVENTION

With reference to the drawing, the device has a first furnace chamber 1which is used for carburising workpieces 2 and which is heated bygas-operated radiant heating tubes 3. The latter are connected via theirexhaust gas pipes 4 to a pressure-change device 5.

Carbon dioxide is removed from the exhaust gas of the radiant heatingtubes 3 in the pressure-change device 5. The carbon dioxide enters agenerator 7 via an outlet pipe 6, said generator also being fed withnatural gas via a pipe 8. As the generator 7 is arranged in the furnacechamber, it is heated by the radiant heating tubes 3. The natural gasreacts with the carbon dioxide in the generator 7. The endogas generatedthereby enters the furnace chamber 1 and causes the workpieces 2 to becarburised.

In this manner the exhaust gas from the radiant heating tubes 3 is freedof carbon dioxide. Thus the impact on the environment is reduced. Thecarbon dioxide is also used for the generation of endogas whichincreases the mass transfer coefficient and thus leads to a considerableincrease in the speed of carburisation. This improves thecost-effectiveness of the carburisation process.

A local excess of carbon dioxide and steam may form on the materialsurfaces due to the acceleration of carburisation. Methane is introducedinto the furnace chamber 1 at a suitable point--schematically shown by apipe 9--to buffer said excess.

The device also has a second furnace chamber 10 which is used fornitrocarburising workpieces 11. The second furnace chamber is heated byradiant heating tubes 12. These are also connected by their exhaust gaspipes 13 to the pressure-change device 5. Thus, they also help to supplythe generator 7 with carbon dioxide.

The pressure-change device 5 has an outlet pipe 14 which is used forintroducing the remaining exhaust gas from the radiant heating tubes 3and 12 into the second furnace chamber. The remaining exhaust gas stillcontains some carbon dioxide and also some nitrogen. Together withammonia, which is fed in through a pipe 15, the remaining exhaust gasforms the atmosphere for nitrocarburisation of the workpieces 11.

A control system not shown in the drawing ensures that the carbondioxide content of the exhaust gases in the pressure-change device isdivided between furnace chambers 1 and 10 in accordance with therelevant carbon dioxide requirements. Furthermore, the control systemensures synchronisation of the individual processes such that the entireprocess can be operated continuously.

Modifications are perfectly possible within the scope of the presentinvention. For example, the second furnace chamber 10 can be dispensedwith. Instead the remaining exhaust gas from the pressure-change device5 can be used for inertisation of locks or similar. Furthermore, thegenerator 7 can be arranged outside the furnace chamber 1. However, anadditional heating system is then necessary. The generator 7 can beentirely dispensed with. The carbon dioxide coming from thepressure-change device 11 is under these circumstances fed directly intothe furnace chamber mixed with the natural gas.

I claim:
 1. A method for the heat treatment of workpieces, said methodcomprising the steps of:a) heating said workpieces with radiant heatwhich is generated through the combustion of gaseous fuel, in particularnatural gas; b) subjecting at least some of said workpieces to acarburisation atmosphere; c) separating carbon dioxide out of exhaustgas formed during said combustion of said gaseous fuel; d) mixing saidcarbon dioxide with hydrocarbon-containing gas, in particular naturalgas, to create a gas mixture; and e) heating said gas mixture to producesaid carburisation atmosphere.
 2. The method according to claim 1,wherein said gas mixture is heated with said radiant heat used to heatsaid workpieces.
 3. The method according to claim 1 or 2, wherein saidgas mixture is passed over a catalyst.
 4. The method according to claim1, wherein said carbon dioxide is removed by pressure change from saidexhaust gas formed during said combustion of said gaseous fuel.
 5. Themethod according to claim 1, wherein said carburisation atmosphere isinjected with heavy hydrocarbon.
 6. The method according to claim 1,wherein the rest of said exhaust gas remaining after the removal of saidcarbon dioxide is used for nitrocarburisation of some of saidworkpieces, ammonia being added.